This invention relates to the cooling of turbine buckets and, specifically, to the cooling of the platform region of the bucket, at the base of the airfoil.
Stage one and stage two buckets in gas turbines experience high temperatures and stress, creating low cycle fatigue life concerns, particularly at the leading edge fillet where the airfoil joins the platform at the forward portion of the pressure side of the airfoil.
In aircraft engines, the bucket platforms are cooled by air entering the cavity below the platform, but this arrangement is not completely satisfactory, particularly in connection with the leading edge fillet.
This invention provides a unique solution to the above problem by directing cooling air through a tube, the forward end of which is located closely adjacent the underside of the platform, below the forward portion of the pressure side of the airfoil, so as to effectively impingement cool the platform in the localized region below the leading edge fillet.
In addition, film cooling holes are formed in the platform itself so that air introduced into the cavity by the impingement cooling tube can escape the cavity and flow along the upper surface of the platform along the contour of the pressure side of the airfoil, in a direction toward the trailing edge of the airfoil.
In its broader aspects, the invention relates to a turbine bucket having an airfoil portion and a root portion, with a substantially planar platform at an interface between the airfoil portion and root portion, a platform cooling arrangement comprising at least one bore in the root portion and at least one impingement cooling tube seated in the bore, the tube extending beyond the bore with an outlet in close proximity to a targeted area on an underside of the platform.
In another aspect, the invention relates to a method of cooling a leading edge fillet region of a turbine bucket having an airfoil portion and a root portion, the airfoil portion joined to a platform extending over the root portion, comprising: a) providing a through bore in a leading side of a cover plate in the root portion, the bore communicating with a recess in the root portion underlying the airfoil portion; b) inserting a tube in the bore, with a tip of the tube extending beyond the bore and in close proximity to a targeted area on an underside of the platform; c) supplying cooling air to the recess via the tube such that the targeted area is impingement cooled by the cooling air passing through the tube.